Control circuit for a power supply

Abstract

The invention relates to a control circuit ( 250 ) for a power supply unit ( 200 ) that has an input ( 207, 209 ) for receiving a mains supply ( 208 ), the control circuit ( 250 ) configured to: sample the input ( 207, 209 ) in order to obtain a first sample value; sample the input ( 207, 209 ) in order to obtain a second sample value subsequent to obtaining the first sample value; compare the first and second sample values to provide an outcome; set a delay interval in accordance with the outcome of the comparison of the first and second sample values; and sample the input ( 207, 209 ) in order to obtain a third sample value after the delay interval has elapsed.

The invention claimed is: 
     
       1. A system configured to regulate a power supply, wherein the power supply is configured to receive a mains supply across first and second inputs, the system comprising:
 a first diode having an anode coupled to the first input; 
 a second diode having an anode coupled to the second input; and 
 a control circuit configured to sample either the first input or the second input, based upon whether the first diode or the second diode is conducting, in order to obtain a first sample value and a second sample value subsequent to the first sample value, and compare the first sample value and the second sample value to determine whether a voltage of the power supply has increased between the first sample value and the second sample value, set a delay interval in accordance with the determined voltage, wherein a longer delay interval is applied when the determined voltage has increased between the first sample value and the second sample value, and obtain a third sample value after the delay interval has elapsed, wherein sampling is suspended when it is determined that the mains supply is present. 
 
     
     
       2. The system of  claim 1 , wherein the control circuit is further configured to set the delay interval as a first delay interval if the determined voltage is indicative of the second sample value being smaller than or equal to the first sample value and set the delay interval as a second delay interval if the determined voltage is indicative of the second sample value being larger than the first sample value. 
     
     
       3. The system of  claim 2 , wherein the second delay interval is longer than the first delay interval. 
     
     
       4. The system of  claim 3 , wherein the second delay interval has a duration of more than 10, 32, 64 milliseconds or 0.5 or 1 second, or is greater than or equal to a duration of a quarter or a half of an AC waveform of the mains supply. 
     
     
       5. The system of  claim 1 , further comprising:
 a drainage switch configured to selectively connect the first and second inputs via a resistor to ground in accordance with a control signal, wherein the control circuit is further configured to, when the determined voltage is indicative of the second sample value being smaller than or equal to the first sample value, increment a count of how many consecutive sample values are smaller than or equal to an immediately preceding sample value, when the count is greater than a threshold, set the control signal for closing the drainage switch, and when the count is not greater than the threshold, set the control signal for opening the drainage switch. 
 
     
     
       6. The system of  claim 5 , wherein a duration of the threshold is such that the consecutive sample values are obtained over at least one and a half AC waveforms of the mains supply. 
     
     
       7. The system of  claim 5 , further comprising:
 a current limiting circuit configured to limit a maximum current that can pass through the drainage switch to a limited current level. 
 
     
     
       8. The system of  claim 1 , further comprising:
 a capacitor coupled between the first input and the second input; 
 a drainage switch configured to selectively discharge the capacitor or allow sampling of the first or second inputs in response to receiving a control signal, wherein the control circuit is further configured to provide the control signal to the drainage switch in order to sample the first or second inputs in accordance with the determined voltage. 
 
     
     
       9. The system of  claim 8 , wherein the drainage switch is configured to couple the first and second inputs via a resistor to ground in response to receiving the control signal. 
     
     
       10. The system of  claim 1 , wherein each sample value comprises a metric of the mains supply that is, or is related to, a voltage between the first and second inputs and ground. 
     
     
       11. The system of  claim 1 , wherein the power supply is a battery charger. 
     
     
       12. The system of  claim 1 , wherein a filter capacitor at an input of the power supply is discharged when it is determined that the mains supply is not present. 
     
     
       13. A method for controlling sampling of a power supply configured to receive a mains supply across first and second inputs, comprising:
 sampling, with a control circuit, either the first input or the second input, based upon whether a first diode or a second diode is conducting, in order to obtain a first sample value and a second sample value subsequent to the first sample value; 
 comparing, in the control circuit, the first sample value and the second sample value to determine whether a voltage of the power supply has increased between the first sample value and the second sample value; 
 setting, in the control circuit, a delay interval depending on the determined voltage, wherein a longer delay interval is applied when the determined voltage has increased between the first sample value and the second sample value; and 
 sampling, in the control circuit, the input in order to obtain a third sample value after the delay interval has elapsed, wherein sampling is suspended when it is determined that the mains supply is present. 
 
     
     
       14. The method of  claim 13 , further comprising:
 setting the delay interval as a first delay interval if the determined voltage is indicative of the second sample value being smaller than the first sample value; and 
 setting the delay interval as a second delay interval if the determined voltage is indicative of the second sample value being larger than the first sample value. 
 
     
     
       15. The method of  claim 14 , wherein the second delay interval is longer than the first delay interval. 
     
     
       16. The method of  claim 14 , further comprising:
 considering the second sample value as the first sample value in an immediately subsequent iteration if the delay interval is set as the first delay interval; or 
 considering the third sample value as the first sample value in an immediately subsequent iteration if the delay interval is set as the second delay interval. 
 
     
     
       17. The method of  claim 13 , further comprising:
 discharging a filter capacitor at an input of the power supply when it is determined that the mains supply is not present.